The present invention relates to an inspection apparatus which non-destructively inspects degradation (artificial aging embrittlement) and/or strain damages, or defects in a structure or a practical equipment member by use of a SQUID (superconducting quantum interference device).
One example of the conventional method for detection of the embrittlement (or degradation) of a metal material is disclosed by JP-A-54-61981. In the disclosed method, the presence/absence of the embrittlement of a weld metal of austenitic stainless steel is decided in accordance with whether or not the quantity of .delta. ferrite has been reduced from its initial value by at least 5%.
Non-destructive inspection of defects using SQUID's is discussed by Weinstock et al. in SQUID'85-Superconducting Quantum Interference Devices and their Applications, 853-856, 1985 Walter de Gruyter & Co. , Berlin-New York printed in Germany. The other prior art references relevant to non-destructive inspection of defects using SQUID's include JP-A-63-235876 JP-A-63-32384, JP-A-60-147646 and JP-A-60-58565. These references disclose the use of pickup coils having a higher-order differential coil structure.